The first thing that occurs is why do you need three swipes for two cars? Is it the same swipe for the front doors?
Apart from that, unless there has been some by-law breaching behaviour, you strata committee is over-reaching by demanding that you don’t allow access to your son and his car.
However, if he is permanently parking in a visitor space, that’s another story.
But in summary, you are entitled to write to the committee and ask what the problem is and what is the outcome they desire. If the “problem” is a member of your family coming and going as they please, and using your car space when you aren’t using it, they’d better gird themselves for an embarrassing backdown when NCAT tells them to get back in their box.